Forget about being on the bit for now. At 11, this is not something that should be expected from you. Do you have any past dressage marks you can go by? If so, use these to identify areas you need to work on. If not, just ask an instructor/trainer for ways you can improve. There are so many little things, it's best that you get advice from someone on the ground.
The reason I say forget about being on the bit is because this is actually a high level move when done properly. You can take shortcuts, which many people do due to lack of understanding, but these just cause problems for the horse. It's a great way to work a horse when done as it's meant to be.